Add support for running action on macOS

[ffried]

Currently, running the action on macOS runners fails (the tar.gz is not found). Installing swiftly on macOS uses a pkg installer (see https://www.swift.org/install/macos/).
This is also mentioned in the "Installing Swiftly Automatically" docs.
I've adjusted the action to use the pkg installer on macOS (including running the baserunner tests on macOS), as well as using brew for installing any missing dependencies.

[0xTim]

@ffried looks like there's an issue with the macOS script

[ffried]

@0xTim It doesn't run the new script. The tests use @main of the action explicitly, thus as long as this PR is open, it won't pick up the new script (although it picked up the new workflow).

[ffried]

@0xTim I've adjusted the test workflow to use actions/checkout to check out the repo first, then run the local variant of the action. This should allow the new script to be picked up. If needed, I can create a separate PR for this.

[ffried]

Thanks for kicking off the CI again.

The docs seem to be wrong with regards to the macOS paths. They mention ~/local/bin/swiftly, whereas the correct path seems to be ~/.swiftly/bin/swiftly. Fixed that one.

However, regarding the linux container failures, I'm a bit at a loss. It seems to have different opinions on what is ${HOME}.

Installing swiftly in /root/.local/share/swiftly/bin/swiftly...
Creating shell environment file for the user...
/__w/_temp/71cd8daf-956f-47bb-9c98-e1372860c67d.sh: line 14: /github/home/.local/share/swiftly/env.sh: No such file or directory

The first line is printed by swiftly. The last line is the error the shell emits, where $HOME seems to point to /github/home, which is what GitHub sets on the container (which can be observed in the Initialize Containers step logs).
I have no idea why swiftly doesn't pick up that home path correctly.
I've added back the check for the root user that was there in the original code - probably because of that. That should fix the issue on linux containers.

Last but not least, amazon linux seems to ship with an old glibc which is why the checkout fails. I have no idea how to work around that which is why I commented the checkout action there and it's now using main again for testing.

[ffried]

@0xTim Anything I can do to get this moving?